Comment by rich_sasha
2 days ago
The maths of the flying is a little puzzling to me.
Back of the fag pack, let’s say it is light for an electric car and weights 1000kg. I’ll also go on a limb and guess the air flow is actually subsonic - 200m/s would make it 0.6 Ma (still insanely fast!). Even then, that requires 50 kg of air every second. Or 40 m^3. Using everyone’s favorite metric, this empties an Olympic swimming pool in 1 minute. It also requires a MW of power. I mean… that’s a lot??? Though admittedly, only about as much as accelerating 0-60 in 1 sec (50% of that in fact). So the flying would draw as much power as accelerating at full throttle.
Power requirements would go down a lot with slower exhaust speed, but then you need to shift more fluid. 40 m^3 already seems like a lot to me.
I get it’s a neat little demo and they have to get SpaceX into the picture somehow, but still I don’t understand how it’s supposed to add up. Also, what do I know - just a humble physicist.
In the video they claim the air is supersonic at exit, and point out the mach diamonds -- and I think they said 60 kilonewtons of thrust max, which is really a lot. So I think the volume of air would decrease roughly 60% from your estimate? I'm not a physicist.
I'm imagining this in two phases: phase one - you drive around, and excess energy goes to compressing air. Phase two - you use that compressed air which takes very little additional power, so you get to double up on force. I guess likely a 'running mode' where it's using some power to keep suction on.
I'm curious how much time you get at full blast vs 'just' enough to keep the car pinned to a wall, e.g. 1 vs 2 g.
So let’s double the speed - 1.2 Ma. You need half the mass now - 25kg/sec. If you want to have 10 seconds worth of hover - that’s 250kg of gas. Plus the weight of the storage tank. Plus - this is the amounts for hover. Not for any extra force on top of that.
It still doesn’t make much sense to me. And that’s before considering what happens when you get rocket stream hitting a tarmac road. Never mind public roads, this sounds like a major hazard anywhere. Plus the quantity of compressed gas son board (if that’s how it works)… not a pretty crash. At these numbers, imagine 10MJ released all of a sudden - apparently that’s about 2.5kg of TNT.
Surely I’m missing something… as I say, I can’t make sense of it.
Your math is pretty good, Tesla actually gave the storage tank capacity as 250kg
ETA: this is a horrifying amount of energy to be sitting on top of.
7 replies →
what's there to miss? if these ever make it to market their only purpose will be conspicuous consumption for vice-signalling ding dongs
The "pin to a wall" part uses fans, not the cold gas thrusters. It can probably stay there until the battery runs out. The gas thrusters are only needed if you want to go 0-60 in .8s (with the Thrust version), or if you want to fly (with the Rocket version). Probably those only last a few seconds before the air tanks need to recharge.
Mach diamonds in a compressed air jet aren’t surprising. You can get them from a canned compressed air duster: https://hackaday.com/2024/02/18/canned-air-is-unexpectedly-s...
> phase one - you drive around, and excess energy goes to compressing air.
It's an electric car - unless you've got an incredibly long charging cable, there is no "excess energy" when you're driving around.
The question of when to compress the air in the tank is an interesting one. Ideally you’d use the rockets for acceleration, breaking and cornering; which is all the time.
So probably the compressor will run all the time.
You’ll have to do a lot of modelling to work out the best time to use the rockets, my instinct is that fast cornering is the best use of the rockets. Maybe with a boost button to get you off the line fast at the beginning.
From the marketing materials it looks like straight line acceleration is being prioritised tho, so you charge the tank during braking and cornering.
There could be if you started in Denver.
It _does_ go supersonic, otherwise the converging-diverging nozzle doesn't make sense.
You have the answer in your calculations, you just need to take to its logical conclusion. I say its was balsa not cartoon...
> Back of the fag pack,
napkins, and envelopes at least kind of make sense.
Who was ever writing on the back of cigarette packs?
> Who was ever writing on the back of cigarette packs?
British officers planning trench raids in the front lines during WWI. Or, at least, that's where I've been told the expression originates.
Really, though, is it impossible to imagine no one, ever, using a cigarette pack to jot something down? It's a colloquialism. It doesn't even have to be taken literally to make sense. What is gained by questioning it on that level?
> is it impossible to imagine no one, ever, using a cigarette pack to jot something down?
Wouldn't the back have printing on it? I could imagine someone ripping the pack open and writing on the insides, which are blank.
3 replies →
cigarette packs have not been blank on the back since the commenter has been alive
> Really, though, is it impossible to imagine no one, ever, using a cigarette pack to jot something down? It's a colloquialism. It doesn't even have to be taken literally to make sense. What is gained by questioning it on that level?
I took the OP's question to really be a question about the origin of that phrase, and with your answer, I learned something I didn't know before. So I, for one, am happy the question was asked.
2 replies →
Bike shedding.
this is funny because I know so many people who always note down phone numbers and such for the day on cigarette packs. sometimes even carrying a few with them all the time.
in india.
Smokers who didn't have napkins or envelopes handy, presumably.
[dead]
Back of the what ? Shame on you